High resolution intravascular ultrasound transducer assembly having a flexible substrate

ABSTRACT

An ultrasound transducer assembly of the present invention includes a flexible circuit to which an ultrasound transducer array and integrated circuitry are attached during fabrication of the ultrasound transducer assembly. Integrated circuitry and transducer elements are attached to the flexible circuit while the flexible circuit is in a substantially flat shape. The contacts of the transducer elements are positioned on substantially the same plane such that electrical contact with signal and ground lines on the flexible circuit is established without the need for conductive bridges to physically remote electrodes. In an embodiment of the invention wherein the transducer elements are arranged in a cylindrical array, gaps are entirely filled with backing material having a relatively low acoustic impedance. Structure integrity is enhanced and a path to ground facilitated by electrically conductive disks attached to the ends of the transducer assembly.

INCORPORATION BY REFERENCE

The applicants hereby expressly incorporate by reference in theirentirety the description of an "Apparatus and Method for Imaging SmallCavities" described in Proudian et al. U.S. Pat. No. 4,917,097, thedescription of a "Dilating and Imaging Apparatus" described in Eberle etal. U.S. Pat. No. 5,167,233, the description of an "Ultrasound Catheter"described in Eberle et al. U.S. Pat. No. 5,368,037, the description ofan "Apparatus And Method For Detecting Blood Flow In IntravascularUltrasonic Imaging" in O'Donnell et al. U.S. Pat. No. 5,453,575, and thedescription of a "High Resolution Intravascular Ultrasound TransducerHaving a Flexible Substrate" in Eberle et al. U.S. Ser. No. 08/712,576filed on Sep. 13, 1996 which is a continuation of U.S. Ser. No.08/578,226 filed on Dec. 26, 1995.

FIELD OF THE INVENTION

This invention relates to ultrasound imaging apparatuses placed within acavity to provide images thereof of the type described in Proudian etal. U.S. Pat. No. 4,917,097 and more specifically, to ultrasound imagingapparatuses and methods for fabricating such devices on a scale suchthat the transducer assembly portion of the imaging apparatus may beplaced within a vasculature in order to produce images of thevasculature.

BACKGROUND OF THE INVENTION

In the United States and many other countries, heart disease is aleading cause of death and disability. One particular kind of heartdisease is atherosclerosis, which involves the degeneration of the wallsand lumen of the arteries throughout the body. Scientific studies havedemonstrated the thickening of an arterial wall and eventualencroachment of the tissue into the lumen as fatty material builds uponthe vessel walls. The fatty material is known as "plaque." As the plaquebuilds up and the lumen narrows, blood flow is restricted. If the arterynarrows too much, or if a blood clot forms at an injured plaque site(lesion), flow is severely reduced, or cut off and consequently themuscle that it supports may be injured or die due to a lack of oxygen.Atherosclerosis can occur throughout the human body, but it is most lifethreatening when it involves the coronary arteries which supply oxygento the heart. If blood flow to the heart is significantly reduced or cutoff, a myocardial infarction or "heart attack" often occurs. If nottreated in sufficient time, a heart attack often leads to death.

The medical profession relies upon a wide variety of tools to treatcoronary disease, ranging from drugs to open heart "bypass" surgery.Often, a lesion can be diagnosed and treated with minimal interventionthrough the use of catheter-based tools that are threaded into thecoronary arteries via the femoral artery in the groin. For example, onetreatment for lesions is a procedure known as percutaneous transluminalcoronary angioplasty (PTCA) whereby a catheter with an expandableballoon at its tip is threaded into the lesion and inflated. Theunderlying lesion is re-shaped, and hopefully, the lumen diameter isincreased to improve blood flow.

In recent years, a new technique has been developed for obtaininginformation about coronary vessels and to view the effects of therapy onthe form and structure of a site within a vessel rather then merelydetermining that blood is flowing through a vessel. The new technique,known as Intracoronary/Intravascular Ultrasound (ICUS/IVUS), employsvery small transducers arranged on the end of a catheter which provideelectronic transduced echo signals to an external imaging system inorder to produce a two or three-dimensional image of the lumen, thearterial tissue, and tissue surrounding the artery. These images aregenerated in substantially real time and provide images of superiorquality to the known x-ray imaging methods and apparatuses. Imagingtechniques have been developed to obtain detailed images of vessels andthe blood flowing through them. An example of such a method is the flowimaging method and apparatus described in O'Donnell et al. U.S. Pat. No.5,453,575, the teachings of which are expressly incorporated in theirentirety herein by reference. Other imaging methods and intravascularultrasound imaging applications would also benefit from enhanced imageresolution.

Transducer backing materials having relatively low acoustic impedanceimprove signal quality in transducer assemblies comprising PZT or PZTcomposites. The advantages of such backing materials are explained inEberle et al. U.S. Pat. No. 5,368,037 the teachings of which areexpressly incorporated in their entirety herein by reference. It is alsoimportant to select a matching layer for maximizing the acousticperformance of the PZT transducers by minimizing echoes arising from theultrasound assembly/blood-tissue interface.

When designing a very small device for manufacture in large quantitiesit is important to take into consideration practical limitations such asmanufacturability, reliability, resiliency and performance. Theultrasound catheter assembly must produce high quality raw image signalsfor the signal processing system located outside the body within whichthe intravascular ultrasound transducer assembly is inserted forimaging. However, there is an interest in limiting the number of partssince added complexity can increase the manufacturing costs and reducethe yield of the intravascular ultrasound catheter assemblies. Thedevices must be sufficiently resilient to withstand handling duringmanufacture and use.

SUMMARY OF THE INVENTION

It is a general object of the present invention to improve themanufacturability of an intravascular ultrasound transducer assembly.

It is another object of the present invention to decrease the per-unitcost for manufacturing ultrasound transducer assemblies.

If is yet another object of the present invention to increase the yieldof manufactured ultrasound transducer assemblies.

It is a related object to provide enhanced structural integrity of theelectrical connections in the transducer assembly.

It is another object of the present invention to decrease the complexityof the ultrasound transducer assembly.

The above mentioned and other objects are met in a new ultrasoundtransducer assembly, and method for fabricating the ultrasoundtransducer assembly including a PZT substrate with metallic contactsformed directly on the PZT substrate during a pre-fabrication step.

The ultrasound transducer assembly of the present invention includes aflexible substrate having an inner surface to which transducer signallines and a ground line are attached to form a flexible circuit. In apreferred embodiment of the present invention, the flexible substrateprovides the quarter-wave matching layer for the ultrasound transducers.

An ultrasound transducer array and integrated circuitry are attachedduring fabrication of the ultrasound transducer assembly while theflexible substrate is substantially planar (i.e., flat). In accordancewith an aspect of the present invention, the signal electrode and groundelectrode for transducer elements at least partially extend to thesurface of the transducer elements that establishes contact with theinner surface plane of the flexible circuit. As a consequence both theground and signal electrodes can establish direct electrical contactwith corresponding signal and ground pads on the flexible surface.Therefore, conductive bridges between flexible circuit lines andelectrodes located on a physically remote surface of the transducerelements are no longer required.

In a particular embodiment of the invention, after the transducer arrayand integrated circuit chips are attached to the flexible substrate, theflexible substrate is reshaped into a substantially non-planar shapearound a lumen tube to form a substantially cylindrical shape. Inaccordance with another, more particular, aspect of the presentinvention, the spaces within the ultrasound transducer assembly betweenthe lumen tube, the flex circuit, the transducer array and theintegrated circuits are all filled with a backing material characterizedby relatively low acoustic impedance. While the use of backing materialin the area of the integrated circuits may reduce the physical rigidityof the ultrasound transducer assembly, in accordance with yet anotheraspect of the present invention, metal discs are placed upon the lumentube of the assembly and enhance the physical integrity of the device.The metal discs also form part of a path from a ground wire to theground electrodes of the ultrasound transducer array elements.

The integrated circuitry is housed within integrated circuit chips onthe ultrasound transducer assembly. The integrated circuitry is coupledvia a cable to an imaging computer which controls the transmission ofultrasound emission signals transmitted by the integrated circuitry tothe ultrasound transducer array elements. The imaging computer alsoconstructs images from electrical signals transmitted from theintegrated circuitry corresponding to ultrasound echoes received by thetransducer array elements.

The above described new ultrasound transducer assembly and method formaking such a device retains a two-dimensional aspect to the earlystages of ultrasound transducer assembly fabrication which willultimately yield a three-dimensional, cylindrical device. Furthermore,the flexible circuit and method for fabricating an ultrasound transducerassembly according to the present invention facilitate the constructionof individual, physically separate transducer elements in a transducerarray. Finally, the present device eliminates a number of structureswhich contributed to the complexity of the ultrasound transducerassembly and the method for making such a device.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth the features of the present invention withparticularity. The invention, together with its objects and advantages,may be best understood from the following detailed description taken inconjunction with the accompanying drawings of which:

FIGS. 1-1A is a perspective view of the flat sub-assembly of anultrasound transducer assembly incorporating a 64 element ultrasoundtransducer array and integrated circuits mounted to a flexible circuit;

FIG. 2 is a schematic perspective view of the assembled ultrasoundtransducer assembly from the end containing the cable attachment pad;

FIG. 3 is a cross-section view of the ultrasound transducer assemblyillustrated in FIG. 2 sectioned along line 3--3 in the integratedcircuit portion of the ultrasound transducer assembly;

FIG. 4 is a cross-section view of the ultrasound transducer assemblyillustrated in FIG. 2 sectioned along line 4--4 in the transducerportion of the ultrasound transducer assembly;

FIG. 5 is a longitudinal cross-section view of the ultrasound transducerassembly illustrated in FIG. 2 sectioned along line 5--5 and runningalong the length of the ultrasound transducer assembly;

FIG. 5a is an enlarged view of the outer layers of the sectioned view ofthe ultrasound transducer assembly illustratively depicted in FIG. 5;

FIG. 6 is an enlarged and more detailed view of the transducer region ofthe ultrasound transducer assembly illustratively depicted in FIG. 5;

FIG. 6a is a further enlarged view of a portion of the transducer regioncontaining a cross-sectioned transducer;

FIG. 6b is a side view of a single transducer element in accordance witha preferred embodiment of the invention;

FIG. 7 is a perspective view of a lumen tube and discs assembly inaccordance with a preferred embodiment of the present invention;

FIG. 8 is an outline of the generally circular disc which is pressedonto the lumen tube at the transducer array portion of the ultrasoundtransducer assembly;

FIG. 9 is an outline of the generally pentagonal disc which is pressedonto the lumen tube at the electronics portion of the ultrasoundtransducer assembly;

FIG. 10 is a flowchart summarizing the steps for fabricating acylindrical ultrasound transducer assembly embodying the presentinvention;

FIG. 11 is a schematic drawing showing a longitudinal cross-section viewof a mandrel used to form a mold within which a partially assembledultrasound transducer assembly is drawn in order to re-shape the flat,partially assembled transducer assembly into a substantially cylindricalshape and to thereafter finish the ultrasound catheter assembly inaccordance with steps 59-61 of FIG. 10;

FIG. 12 is a schematic drawing of an illustrative example of anultrasound imaging system including an ultrasound transducer assemblyembodying the present invention and demonstrating the use of the deviceto image a coronary artery; and

FIG. 13 is an enlarged and partially sectioned view of a portion of thecoronary artery in FIG. 12 showing the ultrasound transducer assemblyincorporated within an ultrasound transducer probe located in a catheterproximal to a balloon and inserted within a coronary artery.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning now to FIG. 1, an ultrasound transducer assembly isillustratively depicted in its flat form in which it is assembled priorto forming the device into its final, cylindrical form. The ultrasoundtransducer assembly comprises a flex circuit 2, to which the otherillustrated components of the ultrasound transducer assembly areattached. The flex circuit 2 preferably comprises a flexible polyimidefilm layer (substrate) such as KAPTON™ by DuPont. However, othersuitable flexible and relatively strong materials, such as MYLAR(Registered trademark of E. I. DuPont) may comprise the film layer ofthe flex circuit 2. The flex circuit 2 further comprises metallicinterconnection circuitry formed from a malleable metal (such as gold)deposited by means of known sputtering, plating and etching techniquesemployed in the fabrication of microelectronic circuits upon a chromiumadhesion layer on a surface of the flex circuit 2.

The interconnection circuitry comprises conductor lines deposited uponthe surface of the flex circuit 2 between a set of five (5) integratedcircuit chips 6 and a set of sixty-four (64) transducer elements 8 madefrom PZT or PZT composites; between adjacent ones of the five (5)integrated circuit chips; and between the five (5) integrated circuitchips and a set of cable pads 10 for communicatively coupling theultrasound catheter to an image signal processor via a cable (notshown). The cable comprises, for example, seven (7) 43 AWG insulatedmagnet wires, spirally cabled and jacketed within a thin plastic sleeve.The connection of these seven cables to the integrated circuit chips 6and their function are explained in Proudian (deceased) et al. U.S. Pat.No. 4,917,097.

The width "W" of the individual conductor lines of the metalliccircuitry (on the order of one-thousandth of an inch) is relatively thinin comparison to the typical width of metallic circuitry deposited upona film or other flexible substrate. On the other hand, the width of theindividual conductor lines is relatively large in comparison to thewidth of transmission lines in a typical integrated circuit. The layerthickness "T" of the conductor lines between the chips 6 and thetransducer elements 8 is preferably 2-5 μm. This selected magnitude forthe thickness and the width of the conductor lines enables the conductorlines to be sufficiently conductive while maintaining relativeflexibility and resiliency so that the conductor lines do not breakduring re-shaping of the flex circuit 2 into a cylindrical shape.

The thickness of the flex circuit 2 substrate is preferably on the orderof 12.5 μm to 25.0 μm. However, the thickness of the substrate isgenerally related to the degree of curvature in the final assembledtransducer assembly and its acoustic performance. The thin substrate ofthe flex circuit 2, as well as the relative flexibility of the substratematerial, enables the flex circuit 2 to be wrapped into a generallycylindrical shape after the integrated circuit chips 6 and thetransducer elements 8 have been mounted and formed and then attached tothe metallic conductors of the flex circuit 2. Therefore, in otherconfigurations, designs, and applications requiring less or moresubstrate flexibility such as, for example, the various embodimentsshown in Eberle et al. U.S. Pat. No. 5,368,037, the substrate thicknessmay be either greater or smaller than the above mentioned range. Thus, aflexible substrate thickness may be on the order of several (e.g. 5)microns to well over 100 microns (or even greater)--depending upon theflexibility requirements of the particular transducer assemblyconfiguration.

The flex circuit is typically formed into a very small cylindrical shapein order to accommodate the space limitations of blood vessels. In suchinstances the range of diameters for the cylindrically shaped ultrasoundtransducer assembly is typically within the range of 0.5 mm. to 3.0 mm.in an ultrasound catheter for blood vessel imaging. Furthermore, theflex circuit 2 may also be incorporated into larger cylindricaltransducer assemblies or even transducer assemblies having alternativeshapes including planar transducer assemblies where the flexibilityrequirements imposed upon the flex circuit 2 are significantly relaxed.A production source of the flex circuit 2 in accordance with the presentinvention is Metrigraphics Corporation, 80 Concord Street, Wilmington,Mass. 01887.

The integrated circuit chips 6 are preferably of a type described in theProudian et al. U.S. Pat. No. 4,917,097 (incorporated herein byreference) and include the modifications to the integrated circuitsdescribed in the O'Donnell et al. U.S. Pat. No. 5,453,575 (alsoincorporated herein by reference). However, both simpler and morecomplex integrated circuits may be attached to the flex circuit 2embodying the present invention. Furthermore, the integrated circuitarrangement illustrated in FIG. 1 is intended to be illustrative. Thus,the present invention may be incorporated into a very wide variety ofintegrated circuit designs and arrangements contemplated to fall withinthe scope of the invention.

Finally, the flex circuit 2 illustratively depicted in FIG. 1 includes atapered lead portion 11. As will be explained further below, thisportion of the flex circuit 2 provides a lead into a TEFLON (registeredtrademark of E. I. DuPont) mold when the flex circuit 2 and attachedcomponents are re-shaped into a cylindrical shape. Thereafter, the leadportion 11 is cut from the re-shaped flex circuit 2.

Turning to FIG. 2, an illustrative ultrasound transducer assembly isshown in a re-shaped state. This shape is generally obtained by wrappingthe flat, partially assembled ultrasound transducer assembly shown inFIG. 1 into a cylindrical shape by means of a molding process describedbelow. A transducer portion 12 of the ultrasound transducer assemblycontaining the transducer elements 8 is shaped in a cylinder fortransmitting and receiving ultrasound waves in a generally radialdirection in a side-looking cylindrical transducer array arrangement.The transducer portion 12 on which the transducer elements 8 are placedmay alternatively be shaped or oriented in a manner different from thecylinder illustratively depicted in FIG. 2 in accordance withalternative fields of view such as side-fire planar arrays and forwardlooking planar or curved arrays.

An electronics portion 14 of the ultrasound transducer assembly is notconstrained to any particular shape. However, in the illustrativeexample the portions of the flex circuit 2 supporting the integratedcircuit chips 6 are relatively flat as a result of the electricalconnections between the flex circuit 2 and the integrated circuit chips6. Thus the portion of the flex circuit 2 carrying five (5) integratedcircuit chips 6 has a pentagon cross-section when re-shaped (wrapped)into a cylinder. In an alternative embodiment of the present invention,a re-shaped flex circuit having four (4) integrated circuits has arectangular cross-section. Other numbers of integrated circuits andresulting cross-sectional shapes are also contemplated.

FIG. 2 also shows the set of cable pads 10 on the flex circuit 2extending from the portion of the flex circuit 2 supporting theintegrated circuit chips 6. A lumen 16 in the center of the ultrasoundtransducer assembly (within which a guidewire is threaded during the useof a catheter upon which the transducer assembly has been mounted) isdefined by a lumen tube 18 made of a thin radiopaque, conductivematerial such as Platinum/Iridium. The radiopaque material assists inlocating the ultrasound transducer assembly within the body during amedical procedure incorporating the use of the ultrasound transducerassembly. As will be explained further below, the conductive property ofthe lumen tube 18 offers a means for connecting the transducer groundelectrodes to a ground wire included in at least one of the wiresconnected to the cable pads 10.

Spaces in the re-formed ultrasound transducer assembly between theintegrated circuit chips 6, the transducer elements 8 and the lumen tube18 are filled with a backing material 30. In contrast to earlierultrasound catheter assembly designs including a relatively hard carriermaterial such as a rigid encapsulating epoxy, the backing material 30that fills the spaces between the lumen tube 18 and the integratedcircuit chips 6 is relatively soft. This ensures proper acousticperformance in the transducer portion 12 of the ultrasound transducerassembly. While the backing material 30 does not exhibit the rigidity ofthe previously used epoxy, other structures (disks) incorporated intothe new transducer assembly design, described herein below, provideadditional structural support for the integrated circuit chips 6 andreduces manufacturing complexity.

Turning now to FIG. 3, a cross-section view is provided of theultrasound transducer assembly taken along line 3--3 and looking towardthe transducer portion 12 in FIG. 2. The outside of the electronicsportion 14 has a pentagon shape. The circular outline 26 represents theoutside of the transducer portion 12. The flex circuit 2 encompasses thecylindrically shaped ultrasound transducer assembly. The backingmaterial 30 fills the spaces between the integrated circuit chips 6 andthe lumen tube 18. While relatively soft, the backing material 30provides a satisfactory measure of structural support to the integratedcircuit chips 6 in the final assembly of the ultrasound transducerassembly. A disk (not shown in FIG. 3) inserted in one end of theultrasound transducer assembly housing the integrated circuits 6 furtherenhances the structural integrity of the ultrasound transducer assembly.

Turning now to FIG. 4, a view is provided of a cross-section of theultrasound transducer assembly taken along line 4--4 and looking towardthe electronics portion 14 in FIG. 2. The five corners of the pentagonoutline comprising the electronics portion 14 are illustrated in thebackground of the cross-sectional view at line 4--4. The set ofsixty-four (64) transducer elements 8 are displayed in the foreground ofthis cross-sectional view of the transducer portion 12 of the ultrasoundtransducer assembly. The backing material 30, characterized byrelatively low acoustic impedance, fills the space between the lumentube 18 and the transducer elements 8 as well as the gaps betweenadjacent ones of the sixty-four (64) transducer elements 8.

The determination of desirable materials for the backing material 30 isinfluenced by a number of considerations. The backing material 30preferably possesses the ability to highly attenuate ultrasound energyemitted by the transducer elements 8. The backing material 30 alsoprovides sufficient support for maintaining the array of transducerelements 8 in their desired configuration. A suitable material for thebacking material 30 cures in a sufficiently short period of time to meetmanufacturing needs. A number of known materials meeting the abovedescribed criteria for a good backing material will be known to thoseskilled in the art. An example of such a preferred backing materialcomprises a mixture of epoxy, hardener and phenolic microballoonsproviding high ultrasound signal attenuation and satisfactory supportfor the ultrasound transducer assembly.

Having generally described an ultrasound transducer assemblyincorporating the flex circuit in accordance with the present invention,the advantages provided by the flex circuit will now be described inconjunction with the illustrative embodiment The flex circuit 2 providesa number of advantages over prior ultrasound transducer assemblydesigns. The KAPTON substrate of the flex circuit 2 provides acoustic(quarter-wave) matching for the PZT transducer elements 8.

The ease with which the flex circuit 2 may be re-shaped facilitatesmounting, formation and connection of the integrated circuit chips 6 andtransducer elements 8 while the flex circuit 2 is flat, and thenre-shaping the flex circuit 2 into its final state after the componentshave been mounted, formed and connected. The flex circuit 2 is heldwithin a frame for improved handling and positioning while the PZT andintegrated circuits are bonded to complete the circuits. The singlesheet of PZT or PZT composite transducer material is diced intosixty-four (64) discrete transducer elements by sawing or other knowncutting methods. After dicing the transducer sheet, kerfs exist betweenadjacent transducer elements while the flex circuit 2 is in the flatstate. After the integrated circuit chips 6 and transducer elements 8have been mounted, formed and connected, the flex circuit 2 is re-shapedinto its final, cylindrical shape by drawing the flex circuit 2 and themounted elements into a TEFLON mold (described further below).

Also, because the integrated circuits and transducer elements of theultrasound transducer assembly may be assembled while the flex circuit 2is in the flat state, the flex circuit 2 may be manufactured by batchprocessing techniques wherein transducer assemblies are assembledside-by-side in a multiple-stage assembly process. The flat, partiallyassembled transducer assemblies are then re-shaped and fabricationcompleted.

Furthermore, it is also possible to incorporate strain relief in thecatheter assembly at the set of cable pads 10. The strain reliefinvolves flexing of the catheter at the cable pads 10. Such flexingimproves the durability and the positionability of the assembledultrasound catheter within a patient.

Another important advantage provided by the flex circuit 2, is therelatively greater amount of surface area provided in which to lay outconnection circuitry between the integrated circuit chips 6 and thetransducer elements 8. In the illustrated embodiment of the presentinvention, the transducer array includes sixty-four (64) individualtransducer elements. This is twice the number of transducer elements ofthe transducer array described in the Proudian '097 patent. Doubling thenumber of transducer elements without increasing the circumference ofthe cylindrical transducer array doubles the density of the transducerelements. If the same circuit layout described in the Proudian '097 wasemployed for connecting the electronic components in the sixty-four (64)transducer element design, then the density of the connection circuitrybetween the integrated circuit chips 6 and the transducer elements 8must be doubled.

However, the flex circuit 2 occupies a relatively outer circumferenceof: (1) the transducer portion 12 in comparison to the transducerelements 8 and, (2) the electronics portion 14 in comparison to theintegrated circuit chips 6. The relatively outer circumference providessubstantially more area in which to lay out the connection circuitry forthe sixty-four (64) transducer element design in comparison to the areain which to lay out the connection circuitry in the designillustratively depicted in the Proudian '097 patent. As a result, eventhough the number of conductor lines between the integrated circuitchips 6 and the transducer elements 8 doubles, the density of theconductor lines is increased by only about fifty percent (50%) incomparison to the previous carrier design disclosed in the Proudian '097patent having a substantially same transducer assembly diameter.

Yet another advantage provided by the flex circuit 2 of the presentinvention is that the interconnection solder bumps, connecting themetallic pads of the integrated circuit chips 6 to matching pads on theflex circuit 2, are distributed over more of the chip surface, so thesolder bumps only have to be slightly smaller than the previous designhaving only thirty-two (32) transducer elements.

The integrated circuit chips 6 are preferably bonded to the flex circuit2 using known infrared alignment and heating methods. However, since theflex circuit 2 can be translucent, it is also possible to performalignment with less expensive optical methods which include viewing thealignment of the integrated circuit chips 6 with the connectioncircuitry deposited upon the substrate of the flex circuit 2 from theside of the flex circuit 2 opposite the surface to which the integratedcircuit chips 6 are to be bonded.

Turning now to FIGS. 5 and 5a, a cross-sectional view and enlargedpartial cross-sectional view are provided of the ultrasound transducerassembly illustrated in FIG. 2 sectioned along line 5--5 and runningalong the length of the ultrasound transducer assembly embodying thepresent invention. A KAPTON substrate 33 portion of the flex circuit 2,approximately 13 μm in thickness, completely surrounds the ultrasoundtransducer assembly, acts as an acoustic matching layer and protects theelectronic components of the ultrasound transducer assembly. Metallictransducer signal lines 34, approximately 2-5 μm in thickness, arebonded to the KAPTON substrate 33 with a chromium adhesion layer to formthe flex circuit 2.

The transducer signal lines 34 of the flex circuit 2 are illustrated asa solid layer in FIG. 5. However, it will be appreciated by thoseskilled in the art that the transducer signal lines 34 are fabricatedfrom a solid layer (or layers) of deposited metal using well known metallayer selective etching techniques such as masking or selective platingtechniques.

A cable 35 of the type disclosed in the Proudian '097 patent isconnected to the cable pads 10 for carrying control and data signals theultras between the ultrasound transducer assembly and a processing unit.A set of solder bumps such as solder bump 36 connect the contacts of theintegrated circuit chips 6 to the transducer signal lines 34 of the flexcircuit 2. Two-part epoxy 38 bonds the integrated circuit chips 6 to theflex circuit 2.

FIG. 5 also shows the backing material 30 which fills the gaps betweenthe integrated circuits and the lumen tube 18. The lumen tube 18 has adiameter of approximately 0.024" and is approximately 25 μm thick. Thespace between the transducers 8 and the lumen tube 18 in transducerportion 12 of the ultrasound transducer assembly is filled by thebacking material 30 having a low acoustic impedance and therefore wellsuited for attenuating ringing in the ultrasound transducer assembly byabsorbing ultrasound waves emitted by the transducer elements toward thelumen tube 18. The transducer portion 12 of the ultrasound transducerassembly of the present invention is described in greater detail belowin conjunction with FIGS. 6 and 6a.

A pair of grounding discs 37 and 39 are located on each end of theultrasound transducer assembly. The primary function of the discs 37 and39 is to provide a ground contact between a ground wire on the cable 35,the lumen tube 18, and the transducer ground electrode leads. In thepreferred embodiment of the present invention, mechanical contacts(rather than solder) exist between the transducer ground electrode padsand the disc 37, the disc 37 and the lumen tube 18, the lumen tube 18and disc 39, and disc 39 and a pad on the flex circuit 2 to a groundwire in the cable 35.

The ground contact is established by press-fitting the discs 37 and 39onto the lumen tube 18 as shown in FIG. 7. Thereafter, the flex circuit2 is wrapped around the discs 37 and 39 and the resulting cylindricaldevice is filled with the backing material 30 in order to create adevice having a cross-section illustratively depicted in FIG. 5 afterfinal assembly. As illustratively depicted in FIGS. 8 and 9, the disc 37is generally circular (to provide a round cylinder shape to thetransducer portion 12 of the ultrasound transducer assembly), and thedisk 39 is generally pentagonal (to provide a five-sided cylinder shapeto accommodate the arrangement of the five (5) integrated circuit chips6 attached to flex circuit 2 in the electronics portion 14).Furthermore, the discs 37 and 39 are formed with through holes tofacilitate a step of injecting backing material into the ultrasoundtransducer assembly during a preferred fabrication process describedherein below.

Turning now to FIGS. 6, 6a and 6b, the transducer elements 8 comprisePZT or PZT composite 40 approximately 90 μm in thickness and, dependingon frequency, approximately 40 μm wide and 700 μm long. Each transducerelement includes a Cr/Au ground electrode 42 and a Cr/Au signalelectrode 46 which are approximately 0.1 μm in thickness. Asillustratively depicted in FIG. 6b, the electrodes are constructed byencapsulating the PZT or PZT composite 40 in Cr/Au. Thereafter, theelectrodes 42 and 46 are defined as two separate metal sheets by cutting(or etching) a first groove at point X on a first surface primarilycontaining the signal electrode 46 and cutting a second groove at pointY on a second surface primarily containing the ground electrode 42. Thegrooves at points X and Y define the active region 45 of the transducers8. The reduced active region 45, that does not include the ends of thetransducer elements 8 provides edge damping and potentially improvedimage quality.

As illustratively depicted in FIG. 6b, the positions of the grooves Xand Y establish electrical isolation between the electrodes 42 and 46 ina manner such that connections between electrical lines 44 (ground) and34 (signal) and corresponding transducer electrodes 42 and 46 areachieved without fabricating bridges between lead lines on the flexcircuit 2 and the upper surface of the transducers 8 defining the signalelectrode 46. As a consequence of positioning all electrode contacts ona single plane, connections between electrodes 42 and 46, andcorresponding lines 44 and 34 on the flex circuit 2 are preferablyachieved by means of pressure and adhesive materials rather thansoldering or conductive glues. More particularly, in a preferredembodiment, a two-part epoxy 50, approximately 2-5 μm in thicknessoccupies the space between the ground electrode 42 and the KAPTONsubstrate 33 of the flex circuit 2. The two-part epoxy 50 holds thetransducer elements 8 in signal contact with the transducer signal lines34 of the flex circuit 2 while the relative rough surfaces of the PZT orPZT composite 40 establish several points of contact between thetransducer electrodes 42 and 46, and corresponding electrical lines 44and 34.

The thickness of the two-part epoxy 50 between the substrate 33 and theground electrode 42 is controlled by spacer bars 49. The spacer bars 49run the entire width of the flat flex circuit However, the continuousspacer bar material is separated into discrete bars by a saw during thestep of dicing the transducer material into discrete transducer elements8. Additional two-part epoxy 50 is applied at the ends of thetransducers 8.

Finally, it is noted that the transducer signal lines 34 are separate,electrically isolated conductors which terminate at signal contacts 48.The transducer signal lines 34 couple the transducer elements 8 tocorresponding I/O channels of the integrated circuit chips 6. The groundline 44 comprises a continuous conductor is not cut through since theintegrated circuits and the distal portion of the ground line 44 arefixtured at a lower elevation than the transducer array during dicingand maintains the transducer ground electrode 42 for each of thetransducer elements 8 at a common electrical potential established by aground wire within the cable 35. This ground connection is achievedthrough the metallic disc 37 which conducts a ground signal via thelumen tube 18 and disc 39. The disc 39 is connected directly to theground signal which originates from the cable 35.

Turning now to FIG. 10, the steps are summarized for fabricating theabove-described ultrasound transducer assembly embodying the presentinvention. It will be appreciated by those skilled in the art that thesteps may be modified in alternative embodiments of the invention.

At step 52, the flex circuit 2 is formed by depositing conductivematerials such as Chromium/Gold (Cr/Au) on a surface of the KAPTONsubstrate 33. Chromium is first deposited as a thin adhesion layer,typically 50-100 Angstroms thick, followed by the gold conducting layer,typically 2-5 μm thick. Using well known etching techniques portions ofthe Cr/Au layer are removed from the surface of the KAPTON substrate 33in order to form the transducer signal lines 34, the ground line 44, andthe spacer bars 49 of the flex circuit 2. Also during step 52 goldbumps, used to form the signal contacts 48, are formed on the flexcircuit 2.

In a separate and independent procedure with respect to theabove-described step for fabricating the flex circuit 2, at step 53 athin metal layer, on the order of b 0.1 μm to 5.0 μm is applied to asingle PZT or PZT composite crystal. In contrast to an alternativemetalization procedure, during step 53 the metal layer covers the top,bottom and ends of the PZT crystal. Next, during step 54, the metallayer is divided into two separate metal layers by cutting the twogrooves identified previously by the X and Y in FIG. 6a. These two metallayers will later comprise the separate ground electrode 42 and signalelectrode 46 for each of the transducer elements.

Next, at step 55, the metallized PZT or PZT composite 40 is bonded underpressure to the flex circuit 2 by means of two-part epoxy 50, and curedfor a reasonable period. This is typically done overnight. The pressureexerted during bonding reduces the thickness of the two-part epoxy 50 toa thickness of approximately 2-5 μm, depending on the chosen thicknessof the spacer bars 49 and signal contacts 48. The very thin layer oftwo-part epoxy 50 provides good adhesion of the metallized PZT or PZTcomposite to the flex circuit 2 without significantly affecting theacoustic performance of the transducer elements 8. During exertion ofpressure during step 55, a portion of the two-part epoxy 50 squeezes outfrom between the flex circuit 2 and the transducer sheet from which thetransducer elements 8 will be formed. That portion of the two-part epoxy50 also forms a fillet at each end of the bonded transducer sheet (SeeFIG. 6). The fillets of the two-part epoxy 50 provide additional supportfor the transducer elements 8 during sawing of the PZT or PZT composite40 into physically discrete transducer elements. Additional two-partepoxy 50 may be added around the PZT to make the fillet more uniform.

In order to obtain good performance of the elements and to facilitatere-shaping the flex circuit 2 into a cylinder after the integratedcircuit chips 6 and transducer elements 8 are attached, the transducersheet is diced to form physically discrete transducer elements 8 duringstep 56. Dicing is accomplished by means of a well known high precision,high speed disc sawing apparatus, such as those used for sawing siliconwafers. It is desirable to make the saw kerfs (i.e., the spaces betweenthe adjacent transducer elements) on the order of 15-25 μm when the flexcircuit is re-shaped into a cylindrical shape. Such separationdimensions are achieved by known high precision saw blades having athickness of 10-15 μm.

Continuing with the description of the dicing step 56, after the twopart epoxy 50 is fully cured, the flex circuit 2 is fixtured tofacilitate dicing of the transducer sheet into sixty-four (64) discreteelements. The flex circuit 2 is fixtured by placing the flex circuit 2onto a vacuum chuck (of well known design for precision dicing of verysmall objects such as semiconductor wafers) which is raised by 50-200 μmin the region of the transducer elements 8 in order to enable a sawblade to penetrate the flex circuit 2 in the region of the transducerelements 8 without affecting the integrated circuit region and withoutsawing through the distal portion of the ground line proximate to thedisc 37. The saw height is carefully controlled so that the cut extendscompletely through the PZT or PZT composite 40 and partially into theKAPTON substrate 33 of the flex circuit 2 by a few microns. Extendingthe cut further into the flex circuit 2 further reduces the conductionof ultrasound to adjacent transducer elements The resulting transducerelement pitch (width) is on the order of 50 μm. In alternativeembodiments this cut may extend all the way through the flex circuit 2in order to provide full physical separation of the transducer elements.

Alternatively a laser performs the step of dicing the transducerelements. However, a drawback of using a laser to dice the transducersheet is that the laser energy may depolarize the PZT or PZT composite40. In view of present difficulties associated with polarization of theseparated PZT transducer elements, the sawing method is presentlypreferred.

After the PZT or PZT composite 40 has been diced into discretetransducer elements and cleaned of dust arising from the sawing of thePZT or PZT composite 40, at step 57 the integrated circuit chips 6 areflip-chip bonded in a known manner to the flex circuit 2 using pressureand heat to melt solder bumps such as solder bump 36 forming theelectrical contacts between the flex circuit 2 and the pads of theintegrated circuit chips 6. The integrated circuit chips 6 are alignedby means of either infrared or visible light alignment techniques sothat the Indium solder bumps on the integrated circuits 6 align with thepads on the flex circuit 2. These alignment methods are well known tothose skilled in the art. The partially assembled ultrasound transducerassembly is now ready to be formed into a substantially cylindricalshape as shown in FIGS. 2, 3 and 4.

Before re-shaping the flat flex circuit 2 (as shown in FIG. 1) into acylindrical shape around the lumen tube 18, at step 58 the groundingdiscs 37 and 39 are pressed onto the ends of the lumen tube 18 (see FIG.7). The tolerances of the inner sprockets of the disc 37 and the innerdiameter of the disc 39 and the outer diameter of the lumen tube 18 aresuch that the discs 37 and 39 frictionally engage the outer surface ofthe lumen tube 18. The discs 37 and 39 shown in FIGS. 8 and 9respectively, ensure concentricity of the transducer portion 12 of theassembled ultrasound transducer device around the lumen tube 18 andfacilitates even distribution of the backing material 30 within thespaces of the ultrasound transducer apparatus between the lumen tube andthe ultrasound transducers 8.

At step 59, the grounding assembly, consisting of the lumen tube 18 anddiscs 37 and 39, and the partially assembled flex circuit 2, arecarefully matched up and then drawn into a preformed TEFLON mold havingvery precise dimensions. The TEFLON mold is formed by heat shrinkingTEFLON tubing over a precision machined mandrel (as shown in FIG. 11 anddescribed below). The heat shrinkable TEFLON tubing is removed anddiscarded after fabrication of the ultrasound transducer assembly iscomplete. As a result, distortion of a mold through multiple uses of thesame mold to complete fabrication of several ultrasound transducerassemblies is not a problem, and there is no clean up of the moldrequired.

The TEFLON molds incorporate a gentle lead-in taper enabling the sidesof the flex circuit 2 to be carefully aligned, and the gap between thefirst and last elements to be adjusted, as the flex circuit 2 is pulledinto the mold. In the region of the transducer, the mold and the disc 37are held to a diametric precision of 2-3 μm. Since the flex circuit 2dimensions are formed with precision optical techniques, the dimensionsare repeatable to less than 1 μm, the gap between the first and lastelements (on the outer edges of the flat flex circuit 2) can berepeatable and similar to the kerf width between adjacent elements.

A TEFLON bead is placed within the lumen tube 18 in order to preventfilling of the lumen 16 during the steps described below for completingfabrication of the ultrasound transducer assembly.

After drawing the flex circuit into the mold, at step 60 backingmaterial 30 is injected into the distal end of the ultrasound transducerassembly in order to fill the kerfs between transducer elements and anygaps between the preformed portion of the backing material 30 and thetransducer elements 8. The backing material is injected by means of thethrough holes in the grounding disc 37. The air occupying the spacebetween the lumen tube 18 and components of the flex circuit assemblyescapes through holes in the disc 39. This ensures that there are no airgaps in the region of the ultrasound transducer assembly having thetransducer array since air gaps degrade the performance of theultrasound transducer assembly and degrade the mechanical integrity ofthe device. In contrast to prior fabrication methods employing separateand distinct chip carrier and backing materials, the present designutilizes the backing material 30 to support the integrated circuits.This modification reduces manufacturing complexity while providingsufficient support for the integrated circuits.

At step 61, after the backing material 30 cures, the ultrasoundtransducer assembly is removed from the mold by either pushing thedevice out of the mold or carefully cutting the TEFLON mold and peelingit from the ultrasound transducer assembly. The TEFLON bead is removedfrom the lumen tube 18. Stray backing material is removed from thedevice.

Having described one method for fabricating an ultrasound transducerassembly incorporating the flex circuit 2, it is noted that the order ofthe steps is not necessarily important. For example, while it ispreferred to attach the integrated circuits 6 to the flex circuit 2after the transducers 6 have been bonded to the flex circuit 2, such anorder for assembling the ultrasound transducer assembly is notessential. Similarly, it will be appreciated by those skilled in the artthat the order of other steps in the described method for fabricating anultrasound transducer assembly can be re-arranged without departing fromthe spirit of the present invention.

Turning briefly to FIG. 11, a longitudinal cross-section view isprovided of the mandrel previously mentioned in connection with thedescription of step 59 above The mandrel enables a TEFLON tube to bere-formed into a mold (shown generally by a ghost outline) having veryprecise inside dimensions by heat shrinking the TEFLON tube onto themandrel. The TEFLON mold is thereafter used to re-shape the partiallyassembled ultrasound transducer assembly during step 59. While precisedimensions and tolerances are provided on the drawing, they are notintended to be limiting since they are associated with a particular sizeand shape for an ultrasound transducer assembly embodying the presentinvention.

The mandrel and resulting inside surface of the TEFLON mold generallydisplay certain characteristics. First, the mandrel incorporates a taperfrom a maximum diameter at the end where the flex circuit enters themold to a minimum diameter at the portion of the mold corresponding tothe transducer portion of the ultrasound transducer assembly. This firstcharacteristic facilitates drawing the flex circuit into the mold.

Second, the mold has a region of constant diameter at the region wherethe integrated circuit portion will be formed during step 59. Thisdiameter is slightly greater than the diameter of the transducer regionof the mold where the diameter of the inside surface is precisely formedinto a cylinder to ensure proper mating of the two sides of the flexcircuit when the flat, partially assembled transducer assembly isre-shaped into a cylindrical transducer assembly. The greater diameterin the integrated circuit region accommodates the points of the pentagoncross-section created by the integrated circuit chips 6 when the flatflex circuit is re-shaped into a cylinder.

Finally, a second taper region is provided between the integratedcircuit and transducer portions of the mold in order to provide a smoothtransition from the differing diameters of the two portions.

The above description of the invention has focused primarily upon thestructure, materials and steps for constructing an ultrasound transducerassembly embodying the present invention. Turning now to FIGS. 12 and13, an illustrative example of the typical environment and applicationof an ultrasound device embodying the present invention is provided.Referring to FIGS. 12 and 13, a buildup of fatty material or plaque 70in a coronary artery 72 of a heart 74 may be treated in certainsituations by inserting a balloon 76, in a deflated state, into theartery via a catheter assembly 78. As illustrated in FIG. 12, thecatheter assembly 78 is a three-part assembly, having a guide wire 80, aguide catheter 78a for threading through the large arteries such as theaorta 82 and a smaller diameter catheter 78b that fits inside the guidecatheter 78a. After a surgeon directs the guide catheter 78a and theguide wire 80 through a large artery leading via the aorta 82 to thecoronary arteries, the smaller catheter 78b is inserted. At thebeginning of the coronary artery 72 that is partially blocked by theplaque 70, the guide wire 80 is first extended into the artery, followedby catheter 78b, which includes the balloon 76 at its tip.

After the balloon 76 has entered the coronary artery 72, as in FIG. 13,an ultrasonic imaging device including a probe assembly 84 housed withinthe proximal sleeve 86 of the balloon 76 provides a surgeon with across-sectional view of the artery on a video display 88. In theillustrated embodiment of the invention, the transducers emit 20 MHzultrasound excitation waveforms. However, other suitable excitationwaveform frequencies would be known to those skilled in the art. Thetransducers of the probe assembly 84 receive the reflected ultrasonicwaveforms and convert the ultrasound echoes into echo waveforms. Theamplified echo waveforms from the probe assembly 84, indicative ofreflected ultrasonic waves, are transferred along a microcable 90 to asignal processor 92 located outside the patient. The catheter 78b endsin a three-part junction 94 of conventional construction that couplesthe catheter to an inflation source 96, a guide wire lumen and thesignal processor 92. The inflation and guide wire ports 94a and 94b,respectively, are of conventional PTCA catheter construction. The thirdport 94c provides a path for the cable 90 to connect with the signalprocessor 92 and video display 88 via an electronic connector 98.

It should be noted that the present invention can be incorporated into awide variety of ultrasound imaging catheter assemblies. For example, thepresent invention may be incorporated in a probe assembly mounted upon adiagnostic catheter that does not include a balloon. In addition, theprobe assembly may also be mounted in the manner taught in Proudian etal. U.S. Pat. No. 4,917,097 and Eberle et al. U.S. Pat. No. 5,167,233,the teachings of which are explicitly incorporated, in all respects,herein by reference. These are only examples of various mountingconfigurations. Other configurations would be known to those skilled inthe area of catheter design.

Furthermore, the preferred ultrasound transducer assembly embodying thepresent invention is on the order of a fraction of a millimeter toseveral millimeters in order to fit within the relatively smallcross-section of blood vessels. However, the structure and method formanufacturing an ultrasound transducer assembly in accordance withpresent invention may be incorporated within larger ultrasound devicessuch as those used for lower gastrointestinal examinations.

Illustrative embodiments of the present invention have been provided.However, the scope of the present invention is intended to include,without limitation, any other modifications to the described ultrasoundtransducer device and methods of producing the device falling within thefullest legal scope of the present invention in view of the descriptionof the invention and/or various preferred and alternative embodimentsdescribed herein. The intent is to cover all alternatives, modificationsand equivalents included within the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. An ultrasound transducer assembly forfacilitating providing images from within a cavity, the ultrasoundtransducer assembly comprising:integrated circuitry; an ultrasoundtransducer array including a set of ultrasound transducer elements, eachelement comprising transducer material, and a first electrode formed onat least a first surface and a second surface of the transducer materialand a second electrode formed on at least the second surface of thetransducer material; a flexible circuit to which the ultrasoundtransducer array and the integrated circuitry are attached duringfabrication of the ultrasound transducer assembly, the flexible circuitcomprising:a flexible substrate, providing a re-shapable platform, towhich the integrated circuitry and transducer elements are attached;transducer signal lines on an inner surface of the flexible substrate,the transducer signal lines facilitating an electrical signal pathbetween the integrated circuitry and the set of transducer elements; anda ground line connection on the inner surface of the flexible substrate;andwherein a first contact surface on the first electrode and a secondcontact surface on the second electrode for each transducer element arearranged upon the second surface of the transducer material and uponsubstantially a same physical plane as the inner surface of the flexiblesubstrate.
 2. The ultrasound transducer assembly of claim 1 wherein thefirst electrode is a signal electrode and the second electrode is aground electrode.
 3. The ultrasound transducer assembly of claim 1wherein the transducer elements are covered by a substantiallycontinuous metal layer having first and second notch discontinuitiesthereby defining the signal electrode and ground electrode.
 4. Theultrasound transducer assembly of claim 3 wherein the firstdiscontinuity is located on a surface of each transducer elementprimarily comprising the signal electrode and the second discontinuityis located on a surface of each transducer element primarily comprisingthe ground electrode.
 5. The ultrasound transducer assembly of claim 1wherein the transducer elements include a discontinuity on a metal layeron the second surface of the transducer material, the discontinuityelectrically isolating the first contact surface on the first electrodefrom the second electrode.
 6. The ultrasound transducer assembly ofclaim 1 wherein the flexible substrate provides a quarter-wave matchinglayer for the transducer elements.
 7. The ultrasound transducer assemblyof claim 1 wherein the flexible substrate provides an acoustic matchinglayer for the transducer elements.
 8. The ultrasound transducer assemblyof claim 1 wherein the ultrasound transducer array is substantiallycylindrical in shape.
 9. The ultrasound transducer assembly of claim 8wherein the spaces within the ultrasound transducer assembly between alumen tube, the flex circuit, the transducer array and the integratedcircuits are filled with a backing material characterized by relativelylow acoustic impedance.
 10. The ultrasound transducer assembly of claim9 further comprising at least a first disc attached to the lumen tubeand wherein the outer edges abut the re-shaped flexible substratethereby enhancing the structural integrity of the ultrasound transducerassembly.
 11. The ultrasound transducer assembly of claim 10 wherein thefirst disc comprises a conductive material and wherein the first discprovides a portion of an electrically conductive path between the groundelectrodes and an external ground signal.
 12. The ultrasound transducerassembly of claim 10 wherein the first disc is positioned at an end ofthe transducer assembly housing the transducer array, and a second discattached to the lumen tube is positioned at an opposite end of thetransducer assembly housing the integrated circuitry, and wherein theouter edges abut the re-shaped flexible substrate.
 13. The ultrasoundtransducer assembly of claim 1 wherein the transducer elements arecovered by a substantially continuous metal layer having first andsecond discontinuities thereby defining an active region of eachtransducer element which terminates before an end of the transducermaterial.
 14. The ultrasound transducer assembly of claim 1 wherein thetransducer material comprises a PZT material.
 15. The ultrasoundtransducer assembly of claim 1, having suitable dimensions for providingimages of a blood vessel from within a vasculature, and wherein thediameter of the substantially cylindrical ultrasound transducer assemblyis on the order of 0.3 to 5.0 millimeters.